Ever since it was first formulated a century and a half ago, the Second Law of thermodynamics (or 'law of entropy increase') has had an air of mystery about it. Why is it true? Is it even always true? In this book, Stephen Wolfram builds on recent breakthroughs in the foundations of physics to finally provide a resolution to the mystery of the Second Law, elegantly showing how it emerges as a general feature of processes that can be described computationally and their interplay with our computational characteristics as observers. For Wolfram, the effort to understand the Second Law has been a 50-year quest, beginning when he was 12 years old. In the book, Wolfram tells the story of this quest as well as traces the whole remarkable history of the Second Law. Written with great clarity and richly illustrated with both striking modern diagrams and extensive historical material, the book will be of interest to anyone who wants to understand the foundations and origins of one of the most important and widely applied principles of modern science.